Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session Y1: Fractional Quantum Hall Effect: Spin Effects and Broken-translational-symmetry States |
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Sponsoring Units: DCMP Chair: James Eisenstein, California Institute of Technology Room: Colorado Convention Center Four Seasons 2-3 |
Friday, March 9, 2007 11:15AM - 11:51AM |
Y1.00001: Low frequency spin dynamics in a quantum Hall canted antiferromagnet Invited Speaker: In quantum Hall (QH) systems, Coulomb interactions combined with the macroscopic degeneracy of Landau levels (LLs) drive the electron system into strongly correlated phases as illustrated by the series of fractional QH effects and may also lead to various forms of broken symmetry dictated by the LL filing factor $\nu $. When two layers of such electron systems are closely separated by a thin tunnel barrier, the addition of interlayer interactions and the layer degree of freedom brings about even richer electronic phases, opening up possibilities for different classes of symmetry breaking. In particular, at total filling factor $\nu _{T}$ = 2, where the two of the four lowest LLs split by the Zeeman and interlayer tunnel couplings are occupied, the competing degrees of freedom due to the layer and spin are predicted to lead to rich magnetic phases. Here we present results of resistively detected nuclear spin relaxation measurements in closely separated electron systems that reveal strong low-frequency spin fluctuations in the QH regime at $\nu _{T}$ = 2 [1]. As the temperature is decreased, the spin fluctuations, manifested by a sharp enhancement of the nuclear spin-lattice relaxation rate 1/$T_{1}$, continue to grow down to the lowest temperature of 66 mK. The observed divergent behavior of 1/$T_{1}$ signals a gapless spin excitation mode (i.e., a Goldstone mode) and is a hallmark of the theoretically predicted canted antiferromagnetic order. Our data demonstrate the realization of a two-dimensional system with broken planar spin rotational symmetry, in which fluctuations do not freeze out when approaching the zero temperature limit. [1] N. Kumada, K. Muraki, and Y. Hirayama, Science \textbf{313}, 329 (2006). [Preview Abstract] |
Friday, March 9, 2007 11:51AM - 12:27PM |
Y1.00002: Density dependent anisotropic phases in a two-dimensional hole system Invited Speaker: Anisotropic charge transport is observed in a two-dimensional (2D) hole system in a perpendicular magnetic field at filling factors $\nu $=7/2, $\nu $=11/2, and $\nu $=13/2 at low temperature. In stark contrast, the transport at $\nu $=9/2 is \textit{isotropic} for all temperatures. Our results for a 2D hole system differ substantially from 2D electron transport where no anisotropy has been observed at $\nu $=7/2, and the strongest anisotropy occurs at $\nu $=9/2. Isotropic hole transport at $\nu $=13/2, 11/2 and 7/2 is restored for sufficiently low 2D densities. The density dependence of the observed anisotropies suggests that strong spin-orbit coupling in the hole system contributes to the unusual transport behavior. [Preview Abstract] |
Friday, March 9, 2007 12:27PM - 1:03PM |
Y1.00003: Spectroscopy of quasiparticle excitations in quantum Hall fluids Invited Speaker: Quantum Hall fluids support low-energy excitation modes that are linked to remarkable behaviors that emerge from fundamental interactions in two-dimensions. Inelastic light scattering methods at very low temperatures (below 1 Kelvin) offer unique experimental venues to study excitations in the charge and spin degrees of freedom of the fluids. This talk presents an overview of recent results. The focus is on low-lying excitations that express distinct quantum phases of the electron liquids. The experiments offer insights on translational symmetry, on magnetoroton excitations and on quasiparticle energy level structure. The excitations are probed in diverse states of the electron liquids to provide insights on quasiparticle properties and on the phase transformations between quantum fluid states. [Preview Abstract] |
Friday, March 9, 2007 1:03PM - 1:39PM |
Y1.00004: Microwave Spectroscopy of Wigner crystals in 2DES and Bilayer Systems: Many-body correlation in electronic quantum solids Invited Speaker: It is generally known that in high quality two dimensional electron systems (2DES, similarly for 2D hole systems and bilayer systems) under sufficiently large perpendicular magnetic field $B$, the quantum Hall (QH) states terminate into an electronic solid --- a Wigner crystal (WC) pinned by disorder. After a brief review of solid phases in QH systems (including several recently discovered ones [1]) as known from microwave spectroscopy (measuring a characteristic pinning mode resonance of the solid), I will discuss two of our experiments that highlight the importance of many-body quantum correlation in the high-$B$ WC. In one experiment [2], we measured the \textit{melting} temperature ($T_c$) of the high-$B$ WC at many different $B$ and densities $n$ and in multiple 2DES samples. The data show unambiguously that in a given sample, $T_c$ is controlled by Landau filling $\nu$=$nh/eB$ instead of by $n$. This demonstrates the quantum nature of the high-$B$ WC and that its melting is dependent on many-body quantum correlation (via $\nu$). Such behavior contrasts with any other known solids (in particular, a classical electron solid), whose $T_c$ are determined by $n$. In addition, we found that stronger pinning disorder in samples with tighter vertical confinement led to an enhancement of $T_c$. In another experiment [3], we studied \textit{bilayer} WC (BWC) in bilayer hole systems (in low inter-layer tunneling limit). We found that in samples with a bilayer exciton condensate (BEC) QH state at $\nu$=1, the pinning mode frequency of the BWC ($\nu$$\ll$1) is systematically enhanced from what would be expected from two classically interacting single-layer WC. The enhancement decreases with increasing effective layer separation and is not observed for samples without the $\nu$=1 state. We suggest that our results give evidence for a pseudospin (layer index) ferromagnetic BWC, which possesses interlayer quantum correlation and long range in-plane phase coherence similar to that in the $\nu$=1 BEC state and can experience enhanced pinning [4] in the presence of interlayer spatial correlation of disorder. [1] Yong P.~Chen \textit{et al}., Phys.~Rev.~Lett. \textbf{93}, 206805 (2004); Phys.~Rev.~Lett. \textbf{91}, 016801 (2003); [2] Yong P.~Chen \textit{et al}., Nature Physics \textbf{2}, 452 (2006); [3] Z. Wang \textit{et al}, submitted; [4] Yong P.~Chen, Phys.~Rev.~B \textbf{73}, 115314 (2006). [Preview Abstract] |
Friday, March 9, 2007 1:39PM - 2:15PM |
Y1.00005: Interweaving of Spin and Pseudospin in Bilayer Quantum Hall Systems Invited Speaker: In a bilayer quantum Hall system, the layer index may effectively act like a two-valued degree freedom that is analogous to the spin of the electron. Near filling factor $\nu=1$ this pseudospin is thought to lock into a linear combination of the two possible values of the layer index, yielding an interlayer coherent state analogous to an easy-plane ferromagnet. Such systems possess excitations known as merons, vortex-like objects in which the pseudospin tilts out of the plane near their cores. In quantum Hall systems these are charged objects, and can be injected into the groundstate by doping away from $\nu=1$, yielding a pseudospin textured state. However, recent experiments [1] have suggested that charged excitations may tilt the {\it real} electron spin away from its most polarized state. In this work [2] we study the possibility of simultaneously producing both spin and pseudospin textures in a quantum Hall bilayer near $\nu=1$. Our Hartree-Fock calculations demonstrate that the groundstate generically forms a textured crystal, and that for appropriate choices of Zeeman coupling, interlayer tunneling, interlayer separation, and interlayer bias, the texture can be present in both the spin and pseudospin degrees of freedom. Such states spontaneously break the real rotational spin symmetry and possess a gapless spin wave mode. The possible relevance of this to enhanced NMR relaxation rates observed recently in experiment is discussed. \hfil\break [1] I. Spielman et al., PRL {\bf 94}, 076803 (2005); N. Kumada et al., PRL {\bf 94}, 096802 (2005). \hfil\break [2] J. Bourassa, B. Roostaei, R. C\^ot\'e, H.A. Fertig, and K. Mullen, PRB {\bf 74}, 195320 (2006). [Preview Abstract] |
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